The present invention relates generally to the field of frequency selection elements, and more particularly to a multiple frequency acoustic reflector array and monolithic cover for resonators and method.
Televisions and radios as well as cellular phones and other wireless devices all transmit and/or receive radio frequency signals. Televisions and radios, for example, receive programming from a number of stations in the form of radio frequency signals that are transmitted by the stations. Cellular phones and other two-way wireless communication devices communicate with a base station by both transmitting and receiving radio frequency signals. The radio frequency signals include voice traffic for a wireless telephone connection or data traffic for a wireless Internet or other network connection.
Televisions, radios, cellular phones and other wireless devices are each assigned to different radio frequencies to allow simultaneous operation of the devices within an area. Television, for example, receives signals within the 55 to 800 megahertz (MHz) range while radio receives signals within the 530 to 1,700 kilohertz (kHz) range for AM and within the 88 to 108 megahertz (MHz) range for FM. Cellular phones, in accordance with U.S. standards, operate in the 900 and 1800 megahertz (MHz) range.
Televisions, radios, cellular phones, and other wireless devices each use radio frequency filters to separate out unwanted radio frequency traffic from a desired signal, or channel. In particular, televisions and radios use a number of filters to form a tuner that allows each of the received stations to be selectively tuned. Cellular phones operate at a preset frequency range and include filters dedicated to that frequency range. In each case, the filters discriminate between signals based on frequency diversity to provide a stable signal for use by the receiving device.
Radio frequency filters based on resonators are constructed from pairs of inductors and capacitors arranged in parallel, from crystal resonators and from thin film resonators. The inductor and capacitor configuration resonates in a broad range and therefore provides low quality signal discrimination. Crystal and thin film resonators, on the other hand, resonate in a narrow range and therefore provide high quality signal discrimination.
Crystal resonators include two electrodes with a crystal positioned between them and attached to a pair of posts. The air interface provides a required low acoustic impedance for efficient internal reflection and high quality factor performance. Although crystal resonators provide high signal discrimination, they are limited to applications below 500 megahertz (MHz) due to crystal thickness limitations. As a result, crystal resonators are not suitable for cellular and other lower ultra high frequency (UHF) applications in the 300 to 3000 megahertz (MHz) range.
Thin film resonators include two electrodes with a piezoelectric layer positioned between the electrodes. The piezoelectric layer has a thickness that is an acoustic half wavelength of a target frequency of the resonator to provide resonance and thus filtering for the frequency. The thin film resonator is formed on a substrate and includes a low impedance interface for reflection which leads to efficient internal resonance. he low impedance interface may be an etched via interface, an air gap interface or replaced by a distributed acoustic reflector array. Both the etched via interface and the air gap interface use time consuming and expensive processes to form an air space between a bottom electrode and the supporting substrate. The upper electrode is left uncovered to similarly interface with air.
The distributed acoustic reflector array uses a number of alternating high and low acoustic impedance layers each having a thickness that is a quarter of the target wavelength of the target frequency to reflect back the acoustic signal from the resonator. The distributed acoustic reflector array provides reduced cost compared to the air interface methods and provides a solid support for the resonator. Because acoustic reflector arrays are wavelength specific, however, they are typically ineffective for multi-band applications in which filters of varying frequency, hence wavelength, are employed. As a result, etched via and air gap interfaces must typically be used in multi-band applications.
The present invention provides a multiple frequency acoustic reflector array for acoustic resonators and filters that substantially reduces or eliminates the disadvantages and problems associated with previously developed systems and methods. In particular, a distributed acoustic reflector array includes a plurality of disparate reflector layers that each reflect signals at different frequencies to allow multiple frequency resonators and filters to be supported by a single reflector array.
In accordance with one embodiment of the present invention, a radio frequency filter system includes a first acoustic resonator for a first frequency and a second acoustic resonator for a second frequency. An acoustic reflector array is coupled to a lower electrode of the first acoustic resonator and to a lower electrode of the second acoustic resonator. The acoustic reflector array includes a plurality of reflector layers. A first reflective layer is operable to substantially reflect a signal at substantially the first frequency while the second reflective layer is operable to reflect a signal at substantially the second frequency.
More specifically, in accordance with particular embodiments of the present invention, sets of disparate frequency layers may be included in the acoustic reflector array to provide a suitable reflection plane for each of the acoustic resonators. In another embodiment, the reflector layers may incrementally change from the first frequency to the second frequency to provide the suitable reflection plane. In both of these embodiments, the reflector layers may alternate between high and low impedance.
The technical advantages of the present invention includes providing a single-chip transceiver for a cellular phone or other device that processes radio frequency signals. In particular, a multiple band acoustic reflector array is provided to support disparate frequency filters on a single substrate. As a result, signal degradation caused by bond wires and capacitive bond pads for off-chip filters are eliminated. Thus the performance of the filter system is improved.
Another technical advantage of the present invention includes providing a distributed reflector for multiple band acoustic filters and resonators. In particular, a multi-layer acoustic reflector array is provided that reflects signals at disparate frequencies. As a result, multiple band resonators may be solidly mounted to a common underlying structure. Thus, yield and performance are improved. In addition, fabrication costs are reduced due to the elimination of the time consuming and expensive processing required for underlying air interface structures.
Still another technical advantage of the present invention includes providing a distributed reflector cover for an acoustic resonator. In particular, an acoustic reflector array configured for the frequency of resonator is disposed over the resonator to protect the resonator from humidity and particulates. As a result, operating conditions are optimized for the acoustic resonator and performance losses due to contamination are minimized. other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.